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Experiment 9
MOSFET as a switch and as an amplifier
1.
OBJECTIVE



2.
Verify that a MOSFET can work as a switch which is controlled by the gate voltage.
Experimentally test the switching action of an n-MOSFET.
Experimentally test a MOSFET amplifier.
Theory
MOSFET as a switch A MOSFET can create a conducting path across the channel provided there is sufficient voltage
between the gate and the Source (VGS). This voltage is known as threshold voltage (VT). In other
words if VGS > VT then the channel is on and the input signal applied to the Drain would be
transmitted to the Source. The circuit below illustrates the operation.
power_Mbreakn Vout
Vo
R1
M1
VOFF = 0
VAMPL = 100m
FREQ = 1K
AC = 0
1k
Vin
4Vdc
Vg
0
Fig. 9.1
The output (Vout) is a bit smaller than the input Vin because of the channel resistance (RCH) of
the MOSFET. It is given by
RL
Vo 
Vin
(1)
RL  RCH
For VGS < VT, MOSFET is off and RCH = .
3
SIMULATION PROCEDURE
3.1 MOSFET as a switch (Fig. 9.1)1. Using CAPTURE window of PSPICE draw the circuit shown in Fig. 9.1. For the NMOS choose
POWER NMOS from the component list located in the PLACE menu. This transistor is not present
in the EVAL library.
2. Create a simulation profile with the following setting – Analysis Type = TIME DOMAIN; Run to
Time = 5ms
3. Run simulation for Vg = 0, 1, 2, 3, 4 and record the amplitude of the output. Can you now estimate
VT of the MOSFET? Find out VT from the output file which is accessible through the output
window.
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4
EXPERIMENTAL WORK
4.1
To observe switching action in a n-MOSFET (VN2222LL)
1 Assemble the circuit as shown in figure 9.1 and set the input amplitude to 50 mV. The pin
configuration of the transistor is given below in Fig. 9.2.
Pin No. VN2222LL
MOSFET
2
1
Source
2
Gate
3
Drain
Fig. 9.2
Measure Vout with the value of VG = 0, 0.5, 1.0, 1.5, 2.0, and 2.5 V. The output Vout can be
measured on the oscilloscope. Note the gain for every value of Vg.
4.2
To observe amplification using an n-MOSFET (VN2222LL)
Assemble the circuit as in figure 9.3 below.
Vo
R1
n-MOSFET
VN2222LL
33K
V2
30Vdc
Vin
VOFF = 1Vdc
VAMPL = 10mV
FREQ = 1KHz
Fig. 9.3
While assembling this circuit, chose a 33 K resistor and measure its value using the DMM. If you
place a smaller resistor by mistake, the transistor will get burnt. VOFF can be obtained using Function
Generator using voltage offset button. Start with VOFF = 1300 mV and increase this value in the steps
of 50 mV and go up to 1700 mV. (Note – Since transistors are not identical, be prepared to change the
VOFF range specified above so that you do observe amplification). You will observe varying degree of
amplification in this range by measuring Vo. VOFF is nothing but the gate voltage of the transistor and
selecting the proper value insures that the channel is on and suitable area of transfer curve is chosen.
5
1
2
3
Questions –
In section 4.1, for VG = 2V, calculate RCH.
In section 4.1, when Vg = 0, the transistor’s channel if off and therefore, RCH = . Using equation
(1), one would expect Vout to be zero. Why do you still get a non-zero output?
Using the small-signal analysis, calculate the gain in Fig. 9.3 and compare it with the highest value
obtained in section 4.2. Assume that trans-conductance (gm) = 0.5 mS and the output resistance = 1
M. If we increase R1, would the gain go up?
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